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Greetings,
The kind of self-excitation I'm talking about refers to backup alternators
and has nothing to do with looking at certain web sites "for education purposes
only."
The issue is that alternators typically require an outside source of
electricity to bring themselves on line before they can start making
electricity. In the unlikely event that a battery post breaks off, for
example, totally disabling the battery, a backup alternator could not be brought
on line unless it is self-exciting. Also, although it is unrelated to
self-excitation, there is a good chance that the primary alternator
would stop functioning reliably once the battery is no longer
connected to it.
The SD-8 permanent magnet backup alternator from B&C that I've
installed is reported to function without a battery. I have read, but have
not personally verified, that if the SD-8 is activated once during the
preflight or pre-takeoff checks, it will remain excited throughout the
flight. Therefore, if the battery subsequently fails, the SD-8 will
continue functioning. And for the real worry-warts, Bob Nuckolls
has created a relatively simple wiring schematic that will make
the SD-8 self-exciting even if it had not been activated
previously during the flight.
By the way, the type of battery failure I'm talking about here is a
total failure which makes it appear there is no battery connected, not the more
common situation of a "dead" or weak battery. A dead battery, if it
had enough juice to start the airplane at the beginning of the flight,
would probably still have enough to excite and regulate an alternator,
although the alternator's charge rate might be so high that the alternator's
circuit protection could disconnect it.
Best,
Dennis Johnson, Legacy #257, wings on and hoping for first flight within
two months or so
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